Carbon dioxide can be contained in subsurface geological units (formations) by a process known as CO2 sequestration or enhanced hydrocarbon recovery. The CO2 stored underground may originate from carbon emissions or from other naturally occurring sources of CO2. The CO2 is delivered to the storage site via normal transport from pipelines in a liquefied state or can be liquefied on site and is then pumped underground into a geological layer that has adequate porosity and permeability for storing CO2, referred to as a storage reservoir. The storage reservoir acts as a container for storing the CO2 when geological seal layers, e.g., overlying seal, underlying seal, adjacent seal or fault seal, are positioned above and below and adjacent the storage reservoir. The storage reservoir is typically composed of porous rocks with higher porosity and permeability, for example sandstones, carbonates and chalks. The geological seal layer is typically composed of rocks that are less permeable, for example shales, salts of anhydrites, and low porosity limestones of sandstones.
The seal or cap rock will have a certain capacity to hold the CO2 within the storage reservoir without leaking. These properties for containing CO2 largely relate to the seal capacity which is related to a number of variables including but not limited to porosity, permeability, and interfacial tension of the different molecules of various fluids oil, gas CO2 and water in combination, pressure, temperature and geomechanical properties of rock strength. Accordingly, the natural characteristics of the cap rock formation are relied on for sealing and containing the CO2. When the CO2 is injected and stored in the storage reservoir it increases the pressure on geological seal layers, and if the pressure is increased too much it will breach the cap rock by dilation or leakage of mechanically fracturing the rock. A breach in the cap rock will release the CO2 from containment and contaminate other geological zones, which may include the atmosphere, and lead to serious environmental consequences. Accordingly, underground sequestration of CO2, while effective for storing CO2, faces the problem of cap rock fractures or leakage that can allow CO2 to leak from the storage reservoir.
Herein, a method is disclosed that enhances the storage of carbon dioxide underground by balancing the pressure the CO2 applies to the seal rock by pressurizing at least one additional storage reservoir adjacent to the seal rock with the pressure of a fluid directed into that additional storage reservoir. The pressure of the fluid is maintained at generally the same pressure of the CO2 within its storage reservoir or at a pressure larger than the pressure of the CO2 within its storage reservoir. This balance of pressure exerted on the seal layer reduces the occurrence of a breach in the seal layer for longer, safer CO2 storage.